Technical Field
This invention relates to a chopper device, used in a field such as railways, that transforms an input DC voltage into a DC voltage of a predetermined magnitude and outputs the resulting DC voltage.
Background Art
FIG. 31 is a circuit diagram illustrating an example of a conventional three-level chopper device.
In FIG. 31, a breaker 2 constituted by a current interrupting device having a mechanical contact, a fuse, or the like, a reactor 3, and semiconductor switching elements (also simply called “switches” hereinafter) 4 and 5 are connected in series between a positive pole and a negative pole of a DC power source 1. Here, insulated gate bipolar transistors (IGBTs), MOS field effect transistors (MOSFETs), bipolar power transistors, or the like are used for the switches 4 and 5.
A diode 6 and a capacitor 8 are connected in series to respective ends of the switch 4, and a diode 7 and a capacitor 9 are connected in series to respective ends of the switch 5.
Both ends of a series circuit constituted by the capacitors 8 and 9 and a connection point (midpoint) between the capacitors 8 and 9 serve as output terminals of the chopper device. A load 101, including a half-bridge inverter constituted by switches 10 and 11 such as IGBTs and an AC motor 12, is connected between the output terminals.
According to this chopper device, energy stored in the reactor 3 is supplied to the series circuit constituted by the capacitors 8 and 9 by the switches 4 and 5 turning on and off, and a DC voltage at three potentials is outputted to the load 101 from the series circuit. Here, the voltages of the capacitors 8 and 9 are controlled to higher values than the voltage of the DC power source 1, and thus the circuit illustrated in FIG. 31 functions as a three-level boosting chopper device.
Note that the load 101 operates so as to transform the DC voltage into an AC voltage by the switches 10 and 11 of the half-bridge inverter turning on and off and supply the AC voltage to the motor 12.
When a fault occurs in such a chopper device, protective measures are typically taken by turning the switches 10 and 11 of the inverter off and separating the load 101 from the chopper device, and furthermore turning the switches 4 and 5 that implement chopper operations off so that current does not flow in the load 101.
However, when a short-circuit fault where, for example, the one switch 4 in the chopper device is fully conductive occurs, the equivalent circuit illustrated in FIG. 32 is formed. Normal measures in this case are turning the switches 10 and 11 off, separating the load 101, and turning the other switch 5 in the chopper device off.
FIG. 33 illustrates an equivalent circuit in the case where the load 101 has been separated and the switch 5 has been turned off in response to a short-circuit fault in the switch 4.
In this circuit, series resonance current produced by the reactor 3 and the capacitor 9 flows from the DC power source 1 via the short-circuited switch 4, along a path a indicated by the broken line. Because the switch 4 is short-circuited, the series resonance current cannot be controlled. There is thus a risk that the capacitor 9 will be charged to a higher voltage than the voltage of the DC power source 1 (that is, to a voltage greater than the breakdown voltage of the capacitor 9). Particularly, with this type of chopper device, although there are cases where the device operates with the voltage of the capacitor 9 at a lower value than the voltage of the DC power source 1 during normal operations, the voltage of the capacitor 9 may rise to approximately three times the voltage occurring in normal operations when series resonance current flows along the path a indicated in FIG. 33.
Meanwhile, in the case where the chopper device is installed in a rail car, the voltage of the DC power source 1 supplied from a contact wire fluctuates frequently due to pantograph bounce, regenerative driving of other cars, and so on. Particularly, if a DC source voltage fluctuates drastically, there is a risk that the capacitor 9 will reach an even higher voltage due to the series resonance current in the path a. Assuming an electrostatic capacitance of the capacitor is represented by C and the voltage by V, generally, when a voltage that greatly exceeds the breakdown voltage is instantaneously applied to the capacitor, energy equivalent to CV2/2 is released all at once, which causes the capacitor to explode.
Opening the breaker 2 can be thought of as a method for interrupting the series resonance current in the path a illustrated in FIG. 33. However, in the case where the breaker 2 is constituted by a fuse, for example, the fuse can only be melted by a large current and it takes a certain amount of time for the fuse to be completely melted. There is thus a risk that the voltage in the capacitor 9 will rise before the fuse melts. It is thus desirable that the series resonance current be interrupted by the breaker 2 before the voltage of the capacitor 9 rises. However, in the case where the inductance of the reactor 3 is high, a large surge voltage may arise if current flowing in the reactor 3 is forcefully interrupted by the breaker 2, which may damage the switches and the like. The melting time for the fuse must therefore be somewhat long.
Even if the breaker 2 is constituted by a current interrupting device having a mechanical contact, there is a time delay in the operation of the contact, and thus the operation may be too late for the rise in the voltage of the capacitor 9.
Thus as described above, it is difficult to completely prevent the capacitor 9 from being damaged by overvoltage by interrupting the series resonance current using the breaker 2.
Note that Patent Document 1 discloses related art in which, in a chopper device having substantially the same configuration as that illustrated in FIG. 31, a short-circuit fault or the like is detected in the capacitor on the basis of a potential at a midpoint in the capacitor series circuit, and operations of the switches in the chopper device are then limited.